This invention relates to a display device comprising an electro-optical medium positioned two supporting plates in which pixels divided into a plurality of sub-pixels are defined on at least one of the supporting plates by means of picture electrodes.
Display devices of the type described may be used for displaying alpha-numerical information or video images.
A device of the type mentioned in the opening paragraph is described in EP-A 0,316,774. The device shown in this Patent Application is driven in the so-called multiplex mode in which, in a system of perpendicularly crossing selection rows or address rows (row electrodes) and data rows (column electrodes), the drive is effected by alternately energizing the address rows while the information to be written is presented to the data rows. Different transmission levels (grey scales) can be introduced in such a device by dividing the column electrodes into sub-electrodes (for example, in accordance with surface ratios of 4 : 2 : 1).
To be able to introduce a large number of grey scales (for example, more than 50) the number of sub-electrodes for each column increases considerably (5 or more). Although this problem can be obviated to some extent by dividing the row electrodes also into sub-electrodes, as is suggested in EP-A 0,316,774, this leads to other problems such as, for example, a poorer efficiency in the number of grey scales to be realised because one grey scale can be defined in several ways.
Another possibility of adjusting grey scales is the use of the transmission/voltage characteristic curve, notably if it has a gradual slope such as, for example, in display devices based on the twisted nematic liquid crystal effect. However, an accurately identical adjustment, viewed across the entire display device, is a problem due to local variations in, for example, threshold voltages and voltage variations caused by parasitic resistances and capacitances.
A mixed alternative is described in EP-A 0,361,981 in which, for example, each sub-pixel has 4 sub-levels of transmission, for example, 0, 1/3I.sub.0, 2/3I.sub.0 and I.sub.0. However, also in this case there are problems when defining grey scales, dependent on the adjustability of each sub-pixel.
Another drawback is that for a specific number of grey scales the width of the narrowest sub-electrode with respect to that of the widest sub-electrode becomes very small. For 256 grey scales the width of the narrowest sub-electrode is then approximately 1/17 of the whole. This causes problems of a lithographic nature.